The present invention relates to a poly(trimethylene terephthalate) and a process for producing the same. The present invention relates in more detail to a poly(trimethylene terephthalate) excellent in shade, oxidation resistance stability and melt stability, and a process for producing a poly(trimethylene terephthalate) having a molecular weight in a range from low to high even by polymerization on an industrial scale of the order of tons.
A poly(trimethylene terephthalate) (hereinafter abbreviated to PTT) fiber obtained by melt spinning a polycondensation product of a lower alcohol ester of terephthalic acid or terephthalic acid and 1,3-propanediol (also termed trimethylene glycol, 1,3-propanediol being abbreviated to PDO below) has various excellent properties that cannot be obtained from a poly(ethylene terephthalate) (hereinafter abbreviated to PET) fiber, a nylon 6 fiber, and the like fiber, such as an astonishing soft feeling and drapability that have never been observed before, excellent stretchability, low temperature dye-affinity and weathering resistance. Asahi Chemical Corporation has solved many technological problems associated with the polymerization, spinning, processing, development of commercial articles, and the like of a PTT, and has recently put a PTT fiber on the market for the first time in the world under the trade name of Solo.
As explained above, the technologies related to the production of fibers from PTT have been developed to the level of industrialization. However, the following problems related to the polymerization of PTT still remain unsolved: a poly(trimethylene terephthalate) having a high intrinsic viscosity is difficult to obtain by melt polymerization alone when polymerization is carried out on an industrial scale, and the polymer tends to be easily colored. The polymerization of PTT on an industrial scale herein refers to polymerization on the following scales: for batch polymerization, polymerization on a scale of 0.3 t/batch or more, preferably 1 t/batch or more; for continuous polymerization, polymerization on a scale of 10 t/day or more, preferably 50 t/day or more.
For example, polymerization of PTT becomes understandable when the polycondensation reactivity of PTT is compared with that of poly(butylene terephthalate) (hereinafter abbreviated to PBT). For PBT, when polymerization is conducted using a batch-wise polymerizer equipped with a plate-like agitating blade in the presence of a titanium alkoxide catalyst at 260xc2x0 C. as will be explained in Comparative Example 1, the polymerization degree increases with time. The polymerization degree further increases even when the polymer comes to have an intrinsic viscosity (intrinsic viscosity of 1.4 dl/g or more, polymerization degree of about 160) at which raking becomes difficult. In contrast to PBT, when PTT is polymerized even under the same polymerization conditions, the polymerization degree reaches a peak near an intrinsic viscosity of 0.8 dl/g (polymerization degree of about 100). Further extension of the polymerization time lowers the polymerization degree on the contrary. Moreover, such a peaking phenomenon of the intrinsic viscosity (polymerization degree) in the polymerization of PTT becomes more significant when the polymerization scale becomes larger.
In order to use PTT for clothes and industrial fibers, PTT usually must have an intrinsic viscosity of 0.85 dl/g or more in view of the manifestation of the strength. However, production of PTT having such a viscosity by conventional melt polymerization alone on an industrial scale has been impossible. For example, U.S. Pat. No. 5,798,433 and EP 1/046,662 describe a process for producing PTT showing excellent whiteness and an intrinsic viscosity of 0.85 dl/g or more. However, the descriptions of both of these references are based on experiments carried out on a scale of no more than several liters. Expansion of the polymerization scale in the disclosed process to an industrial scale only results in the production of a polymer having problems with quality such as mentioned below. The intrinsic viscosity of the polymer cannot be as high as 0.85 dl/g or more. A polymer obtained by excessively extending the polymerization time is colored, or tends to be colored when used in the air for a long time. The polymer shows a decrease in the molecular weight in a large proportion during melt molding.
In order to establish a technology of polymerizing PTT having an intrinsic viscosity of 0.85 dl/g or more by melt polymerization alone even on an industrial scale, the present inventors have analyzed the cause of the peaking phenomenon of an intrinsic viscosity (polymerization degree) in the melt polymerization of PTT, and clarified the following matter.
Elementary reactions forming the polycondensation reaction of PTT are mainly the following two reactions: 
The forward reaction is a chain growth reaction (formula (a)) effected by removal of PDO of two terminal hydroxyl groups. The backward reactions include a reaction in which the ester portion suffers decomposition with PDO that has not been excluded from the reaction system (backward reaction of the formula (a)), and the thermal decomposition reaction (formula (b)) of the ester portion.
First, a case in which efficient discharge of PDO from the reaction system is achieved is considered. In this case, because the equilibrium is inclined to the forward reaction side, the backward reaction of the formula (a) can be ignored. When the forward reaction alone of the formula (a) is dominant, the intrinsic viscosity monotonously increases with time. For example, the polycondensation reactions of PET and PET approximately correspond to the above reaction. However, for PTT, because the allyl terminal (CH2xe2x95x90CHCH2OOCxcx9c) of the thermal decomposition product is thermodynamically stabilized, the thermal decomposition reaction rate (kd) is significantly large. As a result, the reaction of the formula (b) contributes much in a relatively short period of time as the polycondensation reaction proceeds. The reaction of the formula (b) therefore becomes dominant over the forward reaction of the formula (a). The levelling off phenomenon of the intrinsic viscosity in the melt polymerization of PTT as explained above is estimated to take place due to the dominant reaction of the formula (b).
Next, the case in which efficient discharge of PDO is not achieved will be considered. For example, the case in which the scale of polymerization is extremely increased, that is, polymerization on an industrial scale corresponds thereto. When the polymerization scale is increased, the specific surface area of the reactant drastically decreases. Discharge of PDO therefore becomes difficult, and PDO remaining in the reaction system breaks the ester bond of PTT (backward reaction of the formula (a)). As a result, contribution of the backward reaction of the formula (a) becomes significant, resulting in no increase in the intrinsic viscosity. The significant contribution is estimated be the reason why the peaking phenomenon of an intrinsic viscosity takes place in an early stage when the polymerization scale is increased.
Accordingly, the present inventors have discovered that the following procedures are essential as means for increasing a finally attained polymerization degree of PTT in melt polymerization alone regardless of the polymerization scale: inhibition of the backward reaction of the formula (a) and the reaction of the formula (b), namely, 1) advancement of the polycondensation reaction in a state in which the thermal decomposition reaction less contributes and 2) efficient discharge of PDO from the reaction system.
An object of the present invention is to provide a PTT excellent in shade, oxidation resistance stability and melt stability and having a molecular weight in a range from low to high even when the PTT is prepared by polymerization on an industrial scale. Another object of the present is to provide a polymerization process capable of producing a PTT having a molecular weight in a range from low to high regardless of the charging scale of the monomer in the polymerization of the PTT having excellent properties as mentioned above, more specifically to provide a polymerization process of PTT capable of inhibiting thermal decomposition in the polycondensation reaction and efficiently discharging PDO.